A storage drive includes a non-volatile physical storage media such as a magnetic or optical mechanical disk or a solid state drive. The storage media is operated by a drive controller to read and write data to the media and transfer data to and from drive controller memory. A drive controller may load from firmware and execute an instance of a real time operating system (RTOS).
A network connected drive is deployed on a network, as a blade or other rack card form factor. In addition to providing remote storage, a network connected drive may host a variety of platform services such as a network file system or a distributed data store. To provide these or other additional services, a network connected drive includes a host controller. The host controller and drive controller have separate processors and execute separate operating systems. Typically, the host controller and drive controller exist as isolated subsystems residing in separate packages placed on one or more printed circuit boards. Due to execution of separate operating systems that independently allocate memory, the host controller and drive controller generally do not share a memory address space or physical memory.
Between the host controller and drive controller is a bus, such as a Small Computer System Interface (SCSI) bus, a Serial Attached SCSI (SAS) bus, Serial ATA (SATA) bus, Fibre Channel, or any other protocol bus for transferring commands and exchanging data objects. Either the host controller or the drive controller may transfer data over the bus, depending on whether the data is read or written. Writing data onto the bus requires copying the data from memory. Reading data off of the bus requires copying the data into memory. For example when writing data, the host controller copies data from a host controller memory onto the bus, and then the drive controller copies the data from the bus into a drive controller memory. As such, dispatching either a write command or a read command requires a data object be copied at least twice, which can increase latency.